Evolution of Threading Dislocation Density and Stress in GaN Films Grown on (111) Si Substrates by Metalorganic Chemical Vapor Deposition

We have studied the evolution of threading dislocations (TDs), stress, and cracking of GaN films grown on (111) Si substrates using a variety of buffer layers including thin AlN, compositionally graded Al _x Ga_1-x N (0 ≤ x ≤ 1), and AlN/Al _y Ga_1-y N/Al _x Ga_1-x N (0 ≤ x ≤ 1, y = 0 and 0.25) multilayer buffers. We find a reduction in TD density in GaN films grown on graded Al _x Ga_1-x N buffer layers, in comparison with those grown directly on a thin AlN buffer layer. Threading dislocation bending and annihilation occurs in the region in the graded Al _x Ga_1-x N grown under a compressive stress, which leads to a decrease of TD density in the overgrown GaN films. In addition, growing a thin AlN/Al _y Ga_1-y N bilayer prior to growing the compositionally graded Al _x Ga_1-x N buffer layer significantly reduces the initial TD density in the Al _x Ga_1-x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film. In-situ stress measurements reveal a delayed compressive-to-tensile stress transition for GaN films grown on graded Al _x Ga_1-x N buffer layers or multilayer buffers, in comparison to the film grown on a thin AlN buffer layer, which subsequently reduces the crack densities in the films.     

MBE growth and properties of GaN and AlxGa1−xN on GaN/SiC substrates

The growth of GaN and AlGaN by molecular beam epitaxy (MBE) has been studied using GaN/SiC substrates. The GaN/SiC substrates consisted of ∼3 μm thick GaN buffer layers grown on 6H-SiC wafers by metalorganic vapor phase epitaxy (MOVPE) at Crée Research, Inc. The MBE-grown GaN films exhibit excellent structural and optical properties—comparable to the best GaN grown by MOVPE. Al_xGa_1−xN films (x ∼ 0.06-0.08) and Al_xGa_1−xN/GaN multi-quantum-well structures which display good optical properties were also grown by MBE on GaN/SiC substrates.     

Comparative Study on MOCVD Growth of a-Plane GaN Films on r-Plane Sapphire Substrates Using GaN, AlGaN, and AlN Buffer Layers

In this work, we have comparatively investigated the effects of the GaN, AlGaN, and AlN low-temperature buffer layers (BL) on the crystal quality of a-plane GaN thin films grown on r-plane sapphire substrates. Scanning electron microscopy images of the a-plane GaN epilayers show that using an AlGaN BL can significantly reduce the density of surface pits. The full-width at half-maximum values of the x-ray rocking curve (XRC) are 0.19°, 0.36°, and 0.48° for the films grown using Al_0.15Ga_0.85N, GaN, and AlN BLs, respectively, indicating that an AlGaN BL can effectively reduce the mosaicity of the films. Room-temperature photoluminescence shows that the AlGaN BL results in lower impurity incorporation in the subsequent a-plane GaN films, as compared with the case of GaN and AlN BLs. The higher crystal quality of a-plane GaN films produced by the Al_0.15Ga_0.85N BL could be due to improvement of BL quality by reducing the lattice mismatch between the BL and r-sapphire substrates, while still keeping the lattice mismatch between the BL and epitaxial a-plane GaN films relatively small.     

Heterojunction DDR THz IMPATT diodes based on AlxGa1-xN/GaN material system

Simulation studies are made on the large-signal RF performance and avalanche noise properties of heterojunction double-drift region (DDR) impact avalanche transit time (IMPATT) diodes based on Al_xGa_1-xN/GaN material system designed to operate at 1.0 THz frequency. Two different heterojunction DDR structures such as n-Al_0.4Ga_0.6N/p-GaN and n-GaN/p-Al_0.4Ga_0.6N are proposed in this study. The large-signal output power, conversion efficiency and noise properties of the heterojunction DDR IMPATTs are compared with homojunction DDR IMPATT devices based on GaN and Al_0.4Ga_0.6N. The results show that the n-Al_0.4Ga_0.6N/p-GaN heterojunction DDR device not only surpasses the n-GaN/p-Al_0.4Ga_0.6N DDR device but also homojunction DDR IMPATTs based on GaN and Al_0.4Ga_0.6N as regards large-signal conversion efficiency, power output and avalanche noise performance at 1.0 THz.     

Impact of strain relaxation of AlmGa1−mN layer on 2-DEG sheet charge density and current voltage characteristics of lattice mismatched AlmGa1−mN/GaN HEMTs

An accurate charge control model to investigate the effect of aluminum composition, strain relaxation, thickness and doping of the Al_mGa_1−mN barrier layer on the piezoelectric and spontaneous polarization induced 2-DEG sheet charge density, threshold voltage and output characteristics of partially relaxed Al_mGa_1−mN/GaN HEMTs is proposed. The strain relaxation of the barrier imposes an upper limit on the maximum 2-DEG density achievable in high Al content structures and is critical in determining the performance of lattice mismatched Al_mGa_1−mN/GaN HEMTs. The model incorporates the effects of field dependent mobility, parasitic source/drain resistance and velocity saturation to evaluate the output characteristics of Al_mGa_1−mN/GaN HEMTs. Close proximity with published results confirms the validity of the proposed model.     

Oxidation characteristics of Si0.85Ge0.15 nanowires

Oxidation characteristics of Si_0.85Ge_0.15 nanowires were investigated using transmission electron microscopy (TEM) analyses. Si_0.85Ge_0.15 nanowires were grown in a tube furnace by vapor–liquid–solid (VLS) method and thermally oxidized at 925 °C for 1–8 h. After oxidation, oxide thicknesses were measured using TEM images. Si_0.85Ge_0.15 nanowires showed a thicker oxide than Si nanowires, for the whole range of oxidation time. The oxidation rate of Si_0.85Ge_0.15 nanowires significantly decreased in nanowires with diameters less than 150 nm. Long-term oxidation in Si_0.85Ge_0.15 nanowire resulted in the oxidation of germanium atoms.     

Effect of surface encapsulation and As4 overpressure on Si diffusion and impurity-induced layer disordering in GaAs,AlxGa1-xAs,and AlxGa1-xAs-GaAs quantum well heterostructures

Data are presented demonstrating that the surface encapsulant and the As₄ overpressure strongly affect Si diffusion in GaAs and Al_xGa_1-xAs, and thus are important parameters in impurity-induced layer disordering. Increasing As₄ overpressure results in anincrease in diffusion depth in the case of GaAs, and adecrease in diffusion depth for Al_xGa_1-xAs. In addition, the band-edge exciton is observed in absorption on an Al_xGa_1-xAs-GaAs superlattice that is diffused with Si and is converted to bulk crystal Al_yGa_1-yAs via impurity-induced layer disordering. In contrast, the exciton is not observed in absorption on GaAs diffused with Si in spite of the high degree of compensation. These data indicate that the Si diffusion process, and the properties of the diffused material, are different for GaAs and for Al_xGa_1-xAs-GaAs superlattices converted into uniform Al_yGa_1-yAs (0 ≤y ≤x ≤ 1) via impurity-induced layer disordering with the amphoteric dopant Si.     

Ultraviolet photoluminescence from undoped and zn doped AlxGa1−xN with x between 0 and 0.75

The origin of the abrupt decrease withx both in electron concentration and in mobility of Al_xGa_1−xN in the range ofx between 0.4 and 0.6 was investigated by photoluminescence from epitaxial layers covering the entire range of alloy composition. Band edge luminescence from undoped layers was observed out to anx value of 0.75. Whenx was larger than 0.2, a peak 0.2–0.5 eV below the band edge peak was also observed both from undoped and from Zn doped samples. This is tentatively ascribed to an unidentified acceptor (or acceptors) related to the presence of Al. No luminescence which could be attributed to a deep native donor defect was observed in semi-insulating Al_xGa_1−xN layers. When Zn was added in the lowx range of the alloys, a broad band 0.5–0.8 eV below the band edge peak was observed as well as a relatively narrow peak.     

Study of grown-in defects and effect of thermal annealing in Al0.3Ga0.7As and GaAs LPE layers

Studies of the grown-in deep-level defects in the undoped n-Al_xGa_1-xAs (x = 0.3) and GaAs epitaxial layers prepared by the liquid phase epitaxy (LPE) techniques have been made, using DLTS, I-V and C-V measurements. The effect of 300 °C thermal annealing on the grown-in defects was investigated as a function of annealing time. The results showed that significant reduction in these grown-in defects can be achieved via low temperature thermal annealing process. The main electron and hole traps observed in the Al_0.3Ga_0.7As LPE layer were due to the E_c-0.31 eV and E_v+0.18 eV level, respectively, while for the GaAs LPE layer, the electron traps were due to the E_c-0.42 and 0.60 eV levels, and the hole traps were due to E_v+0.40 and 0.71 eV levels. Research supported in part by the Air Force Wright Aeronautical Laboratories, Aeropropulsion Lab., Wright Patterson Air Force Base, Ohio, subcontract through SCEEE, contract F33615-81-C-2011, task-4, and in part by AFOSR grant no. 81-0187.     

Characterizations of InxAlyGa1−x−yN alloy systems grown on GaN substrates by molecular-beam epitaxy

In_0.05Al_0.10Ga_0.85N epilayers and Al_0.10Ga_0.90N epilayers have been grown on bulk GaN single crystals and GaN templates by radio-frequency (RF) molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra at different temperatures ranged from 8 to 300 K were measured for these epilayers. The decreasing rates of PL peak intensity of the In_0.05Al_0.10Ga_0.85N epilayers were smaller than those of the Al_0.10Ga_0.90N epilayers. The fluctuations of emission intensities were not observed in the In_0.05Al_0.10Ga_0.85N epilayers by cathodoluminescence observations at 77 K. Our results indicate that In-related effects exist in InAlGaN quaternary alloys on substrates with low-dislocation densities, however, expect that the localization effect related to In-segregation is weak.     

P- and N-type doping of GaN and AlGaN epitaxial layers grown by metalorganic chemical vapor deposition

Mg- and Si-doped GaN and AlGaN films were grown by metalorganic chemical vapor deposition and characterized by room-temperature photoluminescence and Hall-effect measurements. We show that the p-type carrier concentration resulting from Mg incorporation in GaN:Mg films exhibits a nonlinear dependence both on growth temperature and growth pressure. For GaN and AlGaN, n-type doping due to Si incorporation was found to be a linear function of the silane molar flow. Mg-doped GaN layers with 300K hole concentrations p ∼2×10¹⁸ cm⁻³ and Si-doped GaN films with electron concentrations n∼1×10¹⁹ cm⁻³ have been grown. N-type Al_0.10Ga_0.90N:Si films with resistivities as low as p ∼6.6×10⁻³ Ω-cm have been measured.     

Acceptor and donor doping of AlxGa1−xN thin film alloys grown on 6H-SiC(0001) substrates via metalorganic vapor phase epitaxy

Thin films of Si-doped Al_xGa_1−xN (0.03≤x≤0.58) having smooth surfaces and strong near-band edge cathodoluminescence were deposited at 0.35–0.5 μm/h on on-axis 6H-SiC(0001) substrates at 1100°C using a 0.1 μm AlN buffer layer for electrical isolation. Alloy films having the compositions of Al_0.08Ga_0.92N and Al_0.48Ga_0.52N exhibited mobilities of 110 and 14 cm²/V·s at carrier concentrations of 9.6×10¹⁸ and 5.0×10¹⁷ cm⁻³, respectively. This marked change was due primarily to charge scattering as a result of the increasing Al concentration in these random alloys. Comparably doped GaN films grown under similar conditions had mobilities between 170 and ∼350 cm²/V·s. Acceptor doping of Al_xGa_1−xN for x≤0.13 was achieved for films deposited at 1100°C. No correlation between the O concentration and p-type electrical behavior was observed.     

Nitride-based near-ultraviolet multiple-quantum well light-emitting diodes with AlGaN barrier layers

The In_0.05Ga_0.95N/GaN, In_0.05Ga_0.95N/Al_0.1Ga_0.9N, and In_0.05Ga_0.95N/Al_0.18Ga_0.82N multiple-quantum well (MQW) light-emitting diodes (LEDs) were prepared by metal-organic chemical-vapor deposition. (MOCVD). It was found that the 20-mA electroluminescence (EL) intensity of the InGaN/Al_0.1Ga_0.9N MQW LED was two times larger than that of the InGaN/GaN MQW LED. The larger maximum-output intensity and the fact that maximum-output intensity occurred at a larger injection current suggest that Al_0.1Ga_0.9N-barrier layers can provide a better carrier confinement and effectively reduce leakage current. In contrast, the EL intensity of the InGaN/Al_0.18Ga_0.82N MQW LED was smaller because of the relaxation that occurred in the MQW active region of the sample.     

Reduction of deep levels in MOCVD-regrown AlxGa1−x as interfaces by (NH4)2S passivation and in-situ HCl etching

The effects of surface preparation usinginsitu HCl etching and (NH₄)₂S passivation of Al_xGa_1-xAs episurfaces prior to regrowth by MOCVD are analyzed by deep level transient spectroscopy (DLTS), electrochemical profiling (C-V) and room-temperature photoluminescence (PL). Four electron traps were found from the DLTS measurements; a DX-center and three traps previously reported for oxygen-contaminated MOCVD systems. Electrical and optical measurements on quantum well lasers containing a regrown interface in the optical confinement region are also presented. It is demonstrated that both (NH₄)₂S passivation and HClin-situ etching improve the electrical and optical quality of the regrown Al_xGa_1-xAs interfaces, with the best results being obtained when the two methods are used in tandem.     

Dry etching characteristics of In1-x-y Ga x Al y as alloys in CCl2F2:Ar and CH4:H2:Ar discharges

The etching characteristics of InGaAlAs alloys lattice-matched to InP were investigated using low pressure (1 mTorr) electron cyclotron resonance CH₄:H₂:Ar or CCl₂F₂:Ar discharges with additional radiofrequency biasing of the samples. Using CCl₂F₂:Ar discharges with ≥250V negative bias it is possible to obtain equi-rate etching of the material for all compositions between In_0.53Ga_0.47As and In_0.52Al_0.48As. At lower bias values, formation of A1F₃ on the surface leads to an inhibition of the etch rates. By making use of the differential etch rates of InGaAlAs layers of different compositions in CH₄:H₂:Ar mixtures, it is possible to choose dc bias values that allow one to stop the etching at a pre-selected depth in a multi-layer structure. For example, for -150 V bias, one can etch through In_0.53Ga_0.47As, In_0.53Ga_0.40Al_0.07As and Ino.53Ga_0.30Al_0.17As layers, and stop at an underlying layer with composition In_0.53Ga_0.20Al_0.27As.     

AlxGa1−xN/GaN/AlN heterostructures grown on Si(1 1 1) substrates by MBE for MSM UV photodetector applications

Al_xGa_1−xN/GaN/AlN heterostructures on silicon (Si) substrate was developed by nitrogen plasma-assisted molecular beam epitaxy (MBE) and their properties were investigated by scanning electron microscopy (SEM), electron dispersive X-ray (EDX), atomic force microscopy (AFM), high resolution X-ray diffraction (XRD), Raman spectroscopy and Hall effect measurements. High purity gallium (7N) and aluminum (6N5) were used in the Knudsen cells. High purity nitrogen with 7N purity was supplied to radio frequency (RF) source to generate reactive nitrogen species. The nitrogen pressure and a discharge power were kept at 1.5×10⁻⁵ Torr and 300 W, respectively. From SEM measurements, the surface morphology of samples presented 2- and 3-dimensional growth modes. The EDX measurements showed that there were no foreign elements in the grown samples. The HR-XRD measurement has confirmed that the Al_xGa_1−xN/GaN/AlN heterostructures samples were epitaxially grown on Si substrate. All the dominant E₂ phonon modes were found in Raman spectra results. Lastly, Al_xGa_1−xN/GaN/AlN heterostructures based metal–semiconductor–metal (MSM) UV photodetectors were fabricated and the electrical characteristics of the devices were investigated by using current–voltage (I–V) and photo-conductivity measurements. The devices presented good I–V and photoconductivity characteristics.     

Origin of photoluminescence from Er3+ centers in erbium doped GaAs and Al0.4Ga0.6 As grown by MBE

Sharp erbium-related intra-4f shell luminescence from Er doped GaAs and Al_0.4Ga_0.6 As epitaxial layers grown by molecular beam epitaxy (MBE) is presented. The emission arising from the two Er³⁺ excited states,⁴I_13/2 and⁴I_11/2 are studied. We have observed, by means of heat treatment under differentambients such as As, Ga and Al over pressure, that the optically active Er³⁺ preferentially occupies a Column III lattice site or Column III related defects. The photoluminescence results of co-doping Al_0.4Ga_0.6As:Er with Si and Be by MBE is also reported for the first time. A strong single 1.54 μm spontaneous emission line is achieved by co-doping with Be (≈1×10¹⁸ cm⁻³). This improvement is a result of successfully eliminating or suppressing the other transitions without sacrificing the 1.54 μm emission intensity or linewidth.     

Characteristics of Si3N4/GaAs metal-lnsulator-semiconductor interfaces with coherent Si/Al0.3Ga0.7As interlayers

Si₃N₄/GaAs metal-insulator-semiconductor (MIS) interfaces with Si(10Å)/ Al_0.3Ga_0.7As (20Å) interface control layers have been characterized using capacitance-voltage (C-V) and conductance methods. The structure was in situ grown by a combination of molecular beam epitaxy and chemical vapor deposition. A density of interface states in the 1.1 × 10¹¹ eV^-1 cm^-2 range near the GaAs midgap as determined by the conductance loss has been attained with an ex situ solid phase annealing of 600°C in N₂ ambient. A dip quasi-static C-V demonstrating the inversion of the minority-carrier verifies the decent interface quality of GaAs MIS interface. The hysteresis and frequency dispersion of the MIS capacitors were lower than 100 mV, some of them as low as 50 mV under a field swing of about ±2 MV/cm. The increase of the conductance loss at higher frequencies was observed when employing the surface potential toward conduction band edge, suggesting the dominance of faster traps. Self-aligned gate depletion mode GaAs metal-insulator-semiconductor field-effect transistors with Si/Al_0.3Ga_0.7As interlayers having 3 μm gate lengths exhibited a transconductance of about 114 mS/mm. The present article reports the first application of pseudomorphic Si/ Al_0.3Ga_0.7As interlayers to ideal GaAs MIS devices and demonstrates a favorable interface stability.     

Effect of Si doping on the photoluminescence properties of AlGaInP/GaInP multiple quantum wells

The influence of Si doping on the photoluminescence (PL) properties of (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P multiple-quantum-wells (MQWs) was studied. For the samples without p-type layers, the PL peak wavelength from (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P MQWs did not vary when Si was doped in MQWs, the PL peak intensity did not change obviously and the PL FWHM broadened. We consider that Si doping results in worse interface quality of (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P MQWs. However, for the full light-emitting diode (LED) structure samples, the PL intensity of MQWs obviously increased when Si was doped in MQWs. The PL intensity from MQWs with Si-doped barriers was about 13 times stronger than that of undoped MQWs. The PL intensity from MQWs with Si-doped barriers and wells was strong as 28 times as that of undoped MQWs. The reasons are discussed.     

Analysis of PTCDA/ITO Surface and Interface Using X-ray Photoelectron Spectroscopy and Atomic Force Microscopy

利用X射线光电子能谱(XPS)对苝四甲酸二酐[3,4,9,10-perylenetetracarboxylic dianhydride(PTCDA)]/铟锡氧化物(ITO)表面和界面进行了研究.用原子力显微镜(AFM)对PTCDA/ITO样品的表面形貌进行了分析.XPS表明,在原始表面的C1s精细谱存在两个主谱峰和一个伴峰,主谱峰分别由结合能为284.6eV的苝环中的C原子和结合能为288.7eV的酸酐基团中的C原子激发;而结合能为290.4eV的伴峰的存在,说明发生了来源于ITO膜中的氧对C原子的氧化现象.O原子在C=O键和C-O-C键的结合能分别为531.5和533.4eV.在界面处,C1s谱中较高结合能峰消失,且峰值向低结合能方向发生0.2eV的化学位移;O1s谱向低结合能方向发生1.5eV的化学位移.由此可以推断,在界面处PTCDA与ITO的结合是PTCDA中的苝环与ITO中的In空位的结合.AFM的结果显示,PTCDA薄膜为岛状结构,岛的直径约为100～300nm,表面起伏约为14nm.相邻两层PTC-DA分子由于存在离域大π键而交叠和PTCDA分子中的苝环与ITO的In空位的紧密结合是最终导致PTCDA岛状结构形成的原因.